The present invention relates to a connecting system for cables in an electrical connection board, and in particular, to the cross-wiring of cables in an intrinsically safe industrical instrumentation board.
It is common practice in process control operations to associate physically adjacent field devices (i.e., transmitters, I/P converters, solenoid valves, contacts, sensors, etc.) with a common field junction box and to then connect a multiconductor cable to carry the signals to the control system. This technique drastically reduces wiring complexity and saves cost, but unfortunately, it also creates its own problems. For example, on each multicable coming from the field to the control system terminals, there is a specific sequence of devices and also a mixture of heterogeneous signals to be processed. However, most distributed control systems (DCS) are provided with termination units having a fixed array or pattern of homogeneus input functions, e.g. analog input, analog output, low level input, digital input, digital output, etc. Therefore, since sensor signals must be processed into sets of homogeneous functions, the problem arises of directing all multiconductor cables of homogeneous sensors towards the same terminal block so that they may later be connected not only to the various intrinsic safety modules, each of which performs a specific function, but also, to a central processing unit of the process control system.
Since the plurality of sensors and the number of corresponding multiconductor cables is generally large, attempts are made to gather all conductors coming from the same type of sensor onto the same terminal block. However, due to the large number and entanglement of cables, the various types of conductors coming from sensors performing the same function cannot be always directed in a homogeneous manner towards the access terminal block of the board.
Thus, there is a mismatch between the input signal sequences from the field and the termination unit sequences of the control system.
In the past, when a general control board was prepared, it was located in a cabinet of medium to large size and it was also necessary to prepare another cabinet, usually of substantially the same size, in which all intrinsic safety circuitry is located and where all cables coming from the sensors distributed on the process plant must be directed. Thus, by connecting the field multicables and the control system cables to a specially designed cabinet, termed a Marshalling Cabinet, the aforementioned problem was solved. The field multicables are wired to a first row of screw terminals, termed field terminals, and the DCS multicables from the safety barrier cabinet are wired to an opposed row of screw terminals. In this assembly, each field terminal can be hard wired to the corresponding control system terminal as required by the effective process control layout schematic. Thus, the necessary "cross-wiring" to obtain a homogeneous arrangement must be performed in a separate cabinet from the safety barrier cabinet.
Although various coding forms of colors and alphanumeric systems have been studied to simplify this situation, it is presently very onerous to wire all circuits relating to the intrinsically safe circuit modules directly in the same cabinet where the distribution and general-control board for an industrial plant is prepared.
In short, in the present state of the art the assembling necessary to simplify the connections relating to homogeneous functions performed by sensors of the same kind is rather arduous. Consequently, the possibility of safe and immediate access to the conductor or conductors and sensors or series of sensors performing the same function appears very hard not only during the the setting up but also when servicing, changes or repairs are necssary. It is almost always necessary to build a second cabinet designed to accommodate all terminal blocks, connections and intrinsic safety modules, thereby adding to the multiplicity of cables even further. An additional shortfall of the present state of the art is the difficulty involved when interfacing with a computer. In this instance, in addition to the intrinsic safety cabinet and Marshalling cabinet, it is also necessary to use yet another board in order to connect the output to a computer. Thus, the process of data acquisition in an intrinsically safe environment is even more burdensome.
The necessity therefore arises for a particularly simple and reliable assembly capable of providing the desired homogenous connections while avoiding the tangle of calbes created by the assemblies of the prior art.